DRAFT: Close Middleware Spans Early

[jfhbrook-at-work]

Eric suggested that it would be cool if we could close the middleware spans prior to the next middleware getting called. In other words, instead of

GET /
|-mw: x
  |- mw: route
    |- handler: handler

this:

GET /
|-mw: x
|-mw: route
|-handler: handler

This would make traces a lot more compact and give a better idea at a glance of how long the entry for the middleware takes. For exit, we could do another trace on the back, or assume that work is usually short-lived and emit a span event on exit instead.

As you can hopefully see from the diff, this is EXTREMELY hairy. What you won't see from the diff is that the tests currently fail - the test middleware's context gets added to the HTTP span, not the test middleware span. Alas!

I didn't want to push on this too far + instead focus on what we have now, but I thought it was a good idea and wanted to post my notes.

[jfhbrook-at-work]

The bug is that endSpan needs to be spliced after the mw, not before - but finishing this thought will require refactoring how it terminates at the router.

[jfhbrook-at-work]

Since implementing this busted sketch, we learned that there's an alternate bind function which will make implementing this quite a bit easier than using with:

https://medium.com/aspecto/context-managment-in-opentelemetry-nodejs-e5cddf22a09e